For what it’s worth, I have been testing out VoLTE over T-Mobile networks by enabling the VoLTE mode on the Librem 5 modem (which is disabled by default right now). Once enabled, I can successfully receive calls over 4G.
Unfortunately when I attempt to place calls, the carrier disconnects them, sometimes after the other side rings, sometimes before. When in this mode I have to go into modem settings and tell it to be 3G only and then I can place an outgoing call.
This leads me to believe that it could simply be a matter of having the carrier add the modem to the allow list for VoLTE. Or at least that’s the next step we are pursuing. We’ll know a lot more once we get past that point and can test outgoing VoLTE calls once the carrier allows them.